ES2699737T3 - Safety feature - Google Patents

Abstract

Security feature to protect valuable documents, comprising: a luminescent pigment having a host network doped with a first phosphor (L1) and a second phosphor (L2) and which can be optically excited to emit luminescent light, such that the mole fraction of the second phosphor in the luminescent pigment is between 50 ppm and 2000 ppm, such that an emission band of the first phosphor overlaps with an absorption band of the second phosphor and the luminescent pigment is designed in such a way that an excitation energy of the first phosphor (L1), generated by optical excitation of the luminescent pigment, can be transferred to the second phosphor (L2) through an interaction between the first phosphor (L1) and the second phosphor (L2), characterized - by the fraction (x) molar fraction of the first phosphor (L1) in the luminescent pigment and the molar fraction (y) of the second phosphor (L2) in the luminescent pigment are chosen from f This way, an incomplete transfer of the excitation energy from the first to the second phosphor is achieved, such that the luminescent light of the luminescent pigment has a luminescence spectrum with a first luminescence peak (A) emitted by the first luminescent (L1) and a second peak (B) of luminescence emitted by the second phosphor (L2), such that the ratio (P) of the intensity (IB) of the peak of the second peak (B) of luminescence in the sum of the intensities (IA, IB) peak of the first (A) and the second peak (B) of luminescence is at least 20% and at most 80%, preferably at least 30% and at most 70%, particularly preferably at least minus 40% and maximum 60%, and - because the peak wavelengths of the first and second luminescence peaks are shifted to higher wavelengths compared to the optical excitation of the luminescent pigment, and - why the first (L1) and the second phosphor (L2) are chosen n of the rare earth ions erbium, holmium, neodymium, thulium, ytterbium, and - because the host network is realized as an inorganic host network and because the host network is a host network with a garnet structure or with a structure of perovskite is either an oxide or a tungstate or phosphate or niobate or tantalate or silicate or aluminate.

Description

DESCRIPTION

Safety feature

The invention relates to a security feature and to a method for checking the security feature. Such security features are used to protect valuable documents, in particular, to protect the authenticity of valuable documents.

To protect valuable documents, they are provided with security features and / or security elements that present security features to enable a verification of the authenticity of the document of value. Security features and security elements serve to protect the unauthorized reproduction of valuable documents. Security elements, for example, security threads or foil elements that are attached to a valuable document are used as security elements. The security features can be attached to the substrate of the security element or to the substrate of the value document itself.

As a safety feature, for example, luminescent pigments are used which are composed of a host network doped with a phosphor. The optical transitions of the luminophore lead to a luminescence of the luminescent pigment. For verifying the authenticity of a value document presented by the luminescent pigment, it is checked, for example, whether the value document has the desired luminescence and, depending on this, it is decided whether the value document is authentic or not.

The state of the art is known improved safety features with greater brightness, in which the luminescent pigment is not only doped with a phosphor, but additionally an activator that allows an increased luminescence intensity. In particular, the activator allows excitation of the luminophore in ranges of wavelengths in which the luminophore usually can not be excited or can only be excited in a deficient manner. The activator and the luminophore are chosen so that the activator tends to a complete transfer of its excitation energy to the phosphor. The activator is optically excited selectively by corresponding light radiation and then transfers its excitation energy completely to the emitter, which then emits a characteristic luminescence. Based on this you can check the authenticity of the value document. On the contrary, the activator itself does not show any luminescence.

In order to impede the imitation of the documents of value, it is also known to equip them with a security feature that has a more complex characteristic luminescence spectrum. To obtain several luminescence peaks, different luminescent pigments are mixed together, each containing only one luminophore. In contrast, luminescent pigments suitable for security applications, in particular, luminescent pigments that emit luminescence exclusively in the infrared spectral range and are doped with two or more luminiscent luminophores, are not commercially available. The mixing of different luminescent pigments has the disadvantage that the different luminescent pigments of the mixture can be re-segregated, for example, due to different grain sizes or different densities. Said segregation of the luminescent pigments of a security feature can take place, in particular, due to shaking during transport of the security feature or also during the processing of the security feature for its application on the document of value. The segregation results in a non-homogeneity of the pigment mixture, which can lead to unintentional spatial variations of the luminescence of the security feature on the value document. Such variations can lead to an erroneous valuation of authentic value documents or hinder the recognition of counterfeits that exhibit a luminescence that differs from but is similar to real luminescence. In the case of a security feature of this type, which is composed of a mixture of several luminescent pigments, usually it is only possible to achieve homogeneity of the pigment mixture with great effort.

US 2010/163747 A1 discloses materials that are doped with two different luminophores that emit in the IR range.

Therefore, an object of the present invention is to indicate a security feature that allows a reliable differentiation between authentic and forged value documents.

This object is achieved by the objects of the independent claims. Dependent configurations and improvements of the invention are indicated in the dependent claims.

The security feature according to the invention comprises a luminescent pigment having a host network doped with a first phosphor and with a second phosphor. In this sense, at least one area of the host network volume is doped with both the first and the second luminophore. The luminescent pigment can be excited by an optical excitation to emit luminescent light. By means of the optical excitation of the luminescent pigment, an excitation energy of the first luminophore is generated directly or indirectly. The components of the luminescent pigment, in particular, the host network and the first and second luminophore, are chosen so that an excitation energy of the first luminophore can be transferred to the second luminophore. By thus, the excitation energy of the first luminophore, generated by optical excitation of the luminescent pigment, can be transferred from the first to the second luminophore through an interaction between the first and the second luminophore. However, in the case of the luminescent pigment according to the invention, the excitation energy is not transferred completely from the first to the second luminophore, as is the case of the luminescent pigments with energy transfer known up to now, but only partially, ie , only with a considerably reduced probability compared to the known luminescent pigments. The luminescent light emitted by the luminescent pigment according to the invention contains, in addition to a luminescence peak of the second luminophore, also a luminescence peak of the first luminophore. In the case of the known luminescent pigments, the complete transfer of the excitation energy from the first to the second luminophore leads to the fact that, as a result of an optical excitation of the luminescent pigment, only one luminescence of the second luminophore is emitted, while the first luminophore does not it emits absolutely no luminescence.

In the case of the luminescent pigment according to the invention, a partial, ie incomplete, transfer of the excitation energy is achieved by choosing the molar fractions of the first and the second luminophore in the luminescent pigment in a suitable manner. In the case of certain luminescent pigments and certain molar fractions which can be determined empirically, a reduced probability of the transfer of the excitation energy from the first to the second phosphor can be precisely adjusted. For example, the mole fractions of the second luminophore can be precisely chosen so low that the probability of the transfer of the excitation energy from the first to the second luminophore is reduced. This reduced probability of the energy transfer influences the luminescent light emitted so that it, in addition to a luminescence peak of the second luminophore, also exhibits a luminescence peak of the first luminophore. The molar fractions of the first and the second luminophore in the luminescent pigment are chosen in the luminescent pigment according to the invention such that the luminescent light of the luminescent pigment has a luminescence spectrum with a first luminescence peak emitted by the first luminophore and a second luminance peak. peak of luminescence emitted by the second luminophore, such that the peak intensity of the second luminescence peak has a percentage of at least 20% and a maximum of 80% in the sum of the peak intensity of the first and second peak of luminescence. As a result, both the first and second luminescence peak respectively stand out considerably from the spectral basis of the luminescence spectrum. Due to these characteristics, the luminescent pigment is especially suitable for use for a security feature to protect valuable documents.

Preferably, the molar fractions of the first and the second luminophore in the luminescent pigment are chosen such that the percentage of the intensity of the peak of the second luminescence peak is at least 30% and at most 70%, in particular, at least 40% and at most 60%, in the sum of the peak intensity of the first and the second luminescence peak. In this way an improved detection of the luminescence peak with the weaker intensity of the two is achieved.

The first and second luminescence peak preferably have a ratio of intensities determined intrinsically by the composition of the luminescence pigment. Therefore, the intensity ratio remains unaffected by many external influences on the safety feature. In contrast to the security features mentioned at the beginning of a mixture of luminescent pigments, which have the problem of segregation and modifications of the luminescence spectrum linked to it, the security feature according to the invention is less sensitive to influences. external factors on the safety characteristic due to the intrinsically determined relationship of both peak intensities. Also during the processing of the security feature to bind it to the value document, the luminescence spectrum of the security feature remains the same. Therefore, due to the intrinsically determined ratio of intensities it is possible to avoid unwanted spatial variations of the luminescence in a value document and, if desired, achieve a spatially homogeneous luminescence of the value document with a reduced effort. The security feature according to the invention is also advantageous in comparison with the security features known up to now that have spatially separated luminescence ranges, for example security features based on core cores that have different luminescent pigments in the core and in the core. the shell of a particle. This is because, to create core-shell particles, only a limited selection of materials can be used and the controlled manufacture of these particles is very complex. On the contrary, the safety feature according to the invention can be manufactured with much less effort.

Surprisingly it was discovered that in the case of some luminescent pigments, in both ranges of the molar fractions both luminescence peaks are emitted and their peak intensities can be respectively varied through the molar fractions of the first and the second luminophore in the luminescent pigment. Therefore, for the safety features according to the invention, those luminescent pigments are chosen in which the ratio of the two peak intensities can be adjusted precisely through the molar fractions of the first and second luminophore. Minor modifications of the molar fractions can achieve different intrinsically determined intensities of both luminescence peaks. It has been shown that a minimum variation of the mole fraction of the second luminophore has a great influence on the peak intensities of the first and the second luminescence peak. In comparison, a variation of the molar fraction of the first luminophore frequently has only a relatively small influence on the peak intensities of both luminescence peaks. However, the mole fraction of the first luminophore should not be as large as is usually chosen in the case of upconverting luminescent pigments ("upconverter"), since in the case of such high molar fractions, the peak intensity of the first luminescence peak is largely suppressed due to the concentration "quenching". Therefore, in the luminescent pigment according to the invention, the molar fraction of the first luminophore in the luminescent pigment is preferably so low that a concentration deactivation of the first luminescence peak does not occur or only extremely reduced.

The luminescent light emitted by the luminescent pigment according to the invention as a consequence of the optical excitation has a luminescence spectrum having a first luminescence peak emitted by the first luminophore and a second luminescence peak emitted by the second luminophore. That is to say that the first and the second peak of luminescence are generated by two different luminophores with which the host network of the luminescent pigment is doped. Depending on the type of the first and second luminophores selected and depending on the optical excitation, their luminescence can also have other luminescence peaks respectively. In contrast to the safety features known up to now, the first and second luminescence spikes of the safety feature according to the invention are emitted by a single luminescent pigment, not by a mixture of two or more luminescent pigments. In particular, the first and second luminescence peak can result from electronic transitions of two different luminophores, with which the luminescent pigment is doped, so that the first and second luminescence peak result from different electronic transitions of the same luminescent pigment. .

In the case of the luminescent pigments according to the invention, the excitation energy of the first luminophore can be transferred from the first to the second phosphor. For the luminescent pigment according to the invention, for example, combinations of two luminophores having approximately equal energy level separations in the same host network are used, so that, in the case of a transition of the first luminophore at a higher energy level than a lower one, its energy can be absorbed by the second luminophore by its transition from a lower energy level to a higher one. For this purpose, for example, luminescent pigments in which an emission band of the first luminophore overlaps with an absorption band of the second luminophore are suitable. The transfer of the excitation energy can take place, in particular, by a transfer of resonant energy between the first and the second luminophore. Which luminophores in a host network are really capable of an energy transfer can be consulted in the specialized bibliography. In this way, many combinations of two luminophores and the suitable host network, which in principle are suitable for the process according to the invention, can be indicated. Once two luminophores are selected that are capable of an energy transfer in a host network, the average probability of this energy transfer is reduced precisely, for example, by reducing the mole fraction of one or both luminophores in the host network . In practice, the luminescent pigment with the desired luminescence characteristics can be determined by a series of routine tests, in which the molar fractions of the luminophores in the host network are varied. Then, as luminescent pigments according to the invention, one or more luminescent pigments can be used with those molar fractions of the luminophores for which the luminescence peaks of both luminophores clearly stand out in the luminescence spectrum.

The first and second luminophore can be distributed in a partial zone of the host network. In this sense, the spatial distributions of the first and the second luminophore may overlap completely or partially. However, luminescent pigments in which the first and second luminophore are distributed throughout the host network of the luminescent pigment are preferred, since, in this case, the manufacturing effort for the luminescent pigment is reduced.

The first and second luminophore and the host network are chosen so that an optically generated excitation energy of the first luminophore is transferable by an interaction between the first luminophore and the second luminophore of the first to the second luminophore. The optical excitation of the luminescent pigment can be achieved, in particular, by irradiation of the luminescent pigment with light of a suitable spectral range, in which it absorbs the luminescent pigment. By means of this optical excitation, an excitation energy of the first luminophore is generated, which is then partially transferred to the second luminophore. The second luminophore excited in this way can then deliver its excitation energy again by emitting luminescent light. Due to the only partial transfer of the excitation energy, the first luminophore also delivers a part of its excitation energy by emitting luminescent light. The excitation energy of the first luminophore can be generated in the following manner by optical excitation of the luminescent pigment:

Variant 1

The excitation energy of the first luminophore is generated directly by selective optical excitation of the first luminophore. The excitation energy of the first luminophore is then transferred to the second phosphor. For the selective optical excitation of the first luminophore, the luminescent pigment can be irradiated, for example, precisely with one or more absorption wavelengths of the first luminophore. For this purpose, in particular, a narrow spectral band optical excitation can be used.

Variant 2

The excitation energy of the first luminophore is generated - directly - by optical excitation of the host network and subsequent transfer of the excitation energy from the host network to the first luminophore, and then transferred from the first to the second luminophore. If appropriate, the energy generated in the host network can be transferred from the host network partly to the first luminophore and partially directly to the second phosphoroid. For the optical excitation of the host network, the luminescent pigment is irradiated, for example, precisely with one or more absorption wavelengths of the host network. In particular, for this purpose, the host network can be excited optically in a wide spectral band.

These two variants are not mutually exclusive. Depending on the components of the safety feature according to the invention and depending on the chosen optical excitation, the optical excitation can be achieved either through one of the two variants or both variants together. If appropriate, the first and second luminophore can also transfer their excitation energy to each other alternately, ie in both directions, from one to the other. The transfer of the excitation energy of the first luminophore to the second luminophore can be influenced, if necessary, by the host network. The host network can be chosen, for example, precisely such that by its influence the transfer of the excitation energy from the first to the second luminophore is suppressed or stimulated. In addition, the host network can be doped with one or more additional dopants to influence precisely the transfer of the excitation energy.

The invention also relates to a method for detecting the safety feature, in which an optical excitation of the luminescent pigment is performed to optically excite the luminescent pigment to emit luminescent light and the intensities of the first and second luminescence peak contained are detected. in the luminescence spectrum of the luminescent pigment. These detected intensities of the luminescence peaks may be the peak intensities or the spectrally integrated intensity in the respective luminescence peak. In this context, the optical excitation of the luminescent pigment, through which the excitation energy of the first luminophore is generated, takes place according to the first variant mentioned above and / or according to the second variant mentioned above. To detect the safety characteristic, the detected intensities of the first and the second luminescence peak are evaluated, for example to check the authenticity of the safety feature or the authenticity of a safety element, a printing ink or a document of value presenting the security feature. The radiation of the security feature with light and the detection of the intensities, as well as optionally the evaluation, are carried out with a sensor configured for this purpose.

The molar fraction of the luminophore is called the relative quantitative percentage of the luminophore in the luminescent pigment. The molar x fraction of a luminophore is the relative amount of luminophore particles (e.g., atoms, ions) in the total amount of particles presented by the luminescent pigment according to its empirical formula. Therefore, from the concentration parameter, which indicates the amount of luminophore in the empirical formula of the luminescent pigment, the molar fraction of the luminophore is calculated by dividing by the total amount of particles (atoms, ions) indicated in the empirical formula.

In the case of some of the luminescent pigments according to the invention, the luminescence spectrum varies greatly depending on the precise relative percentage of both luminophores. Variations of a percentage of luminophore within the host network can then lead to locally different peak intensities. For this reason it is preferred that the first and second luminophore are distributed homogeneously in the host network. In particular, in the area of the volume of the host network that is doped both with the first and also with the second luminophore, the first and second luminophore are distributed respectively substantially homogeneously.

In the case of some of the known safety features so far, their luminescence spectrum can be simulated by a similar luminescence spectrum of a falsified luminescent pigment. If the deviations of the exact chemical composition of the safety feature only minimally affect the luminescence spectrum obtained, it is not essential to achieve the identical chemical composition of the luminescent pigment to obtain a sufficiently similar luminescence spectrum. Therefore, for the security feature according to the invention, a luminescent pigment is preferably used which shows a qualitative modification of the luminescence spectrum as a function of the mole fraction of the second luminophore. That is, in the case of the luminescent pigment according to the invention, the peak intensities do not scale uniformly, but change the ratio of the peak intensities of the first and second luminescence peak through the change of the mole fraction of the second luminophore. It is also preferred that the peak intensities of the first and second luminescence peak can be inversely modified with each other through a modification of the mole fraction of the second luminophore. In this sense, the inverse modification of the peak intensities results exclusively from the modification of the mole fraction of the second phosphor, while the molar fraction of the first phosphor remains unchanged. Through a modification of the mole fraction of the second luminophore, the luminescence intensity of one of the two luminescence peaks increases at the expense of the other of the two luminescence peaks. This has the advantage that it increases the security against falsifications of the luminescent pigment. Since, even if for imitation the Correct components of the authentic luminescent pigment, in order to obtain a luminescence spectrum close enough to the safety characteristic, the mole fraction of the phosphor should be precisely achieved. An increase in the mole fraction of the second luminophore can lead to a reduction in the peak intensity of the first luminescence peak and increase in the second luminescence peak.

The components of the luminescent pigment, in particular, the host network and the first and second luminophore, are selected so that these - in the case of modified molar fractions - tend to a complete transfer of the excitation energy from the first to the second luminophore , for example, in the case of a correspondingly higher mole fraction of the second phosphor. In the case of a luminescent pigment of this type (not according to the invention), which has the modified molar fractions, the excitation energy would be completely transferred to the second luminophore and, as a consequence, the peak intensity of the first peak of luminescence compared to the peak intensity of the second luminescence peak. A complete transfer of the excitation energy can take place, for example, in the case of a luminescent pigment (not according to the invention), in which the mole fraction of the second phosphor is chosen multiply times greater than the mole fraction of the second luminophore in the luminescent pigment according to the invention. In addition to the luminescent pigments according to the invention, it is usually possible to indicate a luminescent pigment of this type, not according to the invention, with complete or virtually complete transfer of the excitation energy, which differs from the luminescent pigment according to the invention by a molar fraction increased of the second luminophore.

The interaction, through which, in the case of the luminescent pigment according to the invention, the excitation energy of the first luminophore can be transferred to the second luminophore, takes place in the area of the volume of the host network that is doped both with the first, and also with the second luminophore. Since in this case the interaction takes place within an area of the volume of the same material (of the luminescent pigment), advantages are obtained in comparison with the known security features using spatially separated luminescence zones, between which an interaction takes place. . This is because in these, sufficient spatial closeness of the different luminescence zones must be provided. Therefore, the manufacture of said security features is more complex.

In the case of the luminescent pigment according to the invention, the interaction, through which the excitation energy of the first luminophore can be transferred to the second luminophore, is an interaction without radiation between the first and the second luminophore. Therefore, the first and second luminophore are made so that the excitation energy can be transferred from the first to the second luminophore without exchange of photons. In particular, it can be a dipole-dipole interaction between the first and the second luminophore. The radiation-free transfer of the excitation energy is advantageous in comparison with the known safety features which transfer their excitation energy from one to the other by means of a radiant interaction, since, in the case of the latter, the ratio of the intensities of Peak frequently can only vary within a limited range of values. In addition, due to the reduced absorption rate, said safety features, in which exchange takes place by emission and reabsorption of photons, often require high luminophore concentrations and a complex structure or spatial separation of individual luminophores in spatial zones partial, such as layer systems. In the case of the luminescent pigment according to the invention, in addition to the transfer of the excitation energy between the first and the second luminophore through the interaction without radiation, at the same time additionally, a transfer of the energy of the excitation through an exchange of photons.

Preferably, the peak wavelengths of the first and the second luminescence peak are spectrally separated from each other at least 20 nm, more preferably at least 30 nm. Therefore, during the verification of the safety feature, the spectra of both luminescence peaks can be easily differentiated from each other. Preferably, the peak wavelengths of the first and the second luminescence peak are in the near infrared spectral range, in particular, in the spectral range between 750 nm and 2900 nm, preferably between 800 nm and 2200 nm. Precisely the near infrared spectral range is preferred, because these wavelengths are outside the visible spectral range, which allows a discreet use of the safety feature. Depending on which host network and which first and second phosphors are used, the luminescent pigment can be optically excitable for the emission of luminescent light by radiation with light in the ultraviolet or visible spectral range or in the near infrared spectral range. In the luminescence spectrum of the luminescent pigment, the peak wavelength of the first luminescence peak is, in particular, at a wavelength less than the peak wavelength of the second luminescence peak. And the peak wavelengths of the first and second luminescence peak are shifted to longer wavelengths (Stokes emission) compared to the optical excitation of the luminescent pigment. Unlike in the opposite case, when the optical excitation is at a longer wavelength than the luminescence peaks (anti-Stokes emission, such as that presented by, for example, the up-conversion luminescent pigments), this is advantageous because in the case of the Stokes emission higher luminance intensities can be achieved than in the case of the anti-Stokes emission. Therefore, unlike in the case of the up-converting luminescent pigments, in the case of the luminescent pigments according to the invention, a small amount of the luminescent pigment is sufficient to obtain well detectable peak intensities.

The mole fraction of the second luminophore in the luminescent pigment is between 50 ppm and 2000 ppm. The first luminophore and / or the second luminophore are selected from the rare earth ions erbium, holmium, neodymium, thulium, ytterbium.

The host network is made as an inorganic host network, such that the host network is a host network with a garnet structure or with a perovskite structure or is an oxide or a tungstate or phosphate or niobate or tantalate or silicate or aluminate. For example, the host network is a scandium-gallium garnet, an itrioaluminium garnet or a mixed garnet derived from these. If the host network has a garnet structure or a perovskite structure, it preferably also contains one or more of the elements vanadium, chromium, manganese, iron, cobalt or nickel as the absorbent element. The host network can also be an oxide or a tungstate, phosphate, niobate, tantalate, silicate or aluminate. The host network of the luminescent pigment can also be doped additionally with other dopants which do not emit luminescence, for example, dopants necessary for the formation of crystals. The host network can be additionally doped with one or more different luminophores. The luminescent pigment is made, for example, as a powder, whose particles are composed of the doped host network. The particles may have, for example, a grain size in the range of 1 to 20 pm, preferably <6 pm. The characteristics according to the invention are achieved only in the case of certain compositions of the luminescent pigment, ie certain combinations of both luminophores, molar fractions determined from the luminophores and in the case of certain combinations of luminophore and host network. The choice of another luminophore, another molar fraction of the luminophore or other host network generally leads to a luminescent pigment which does not have the characteristics according to the invention.

The luminescent pigment can also have more than two luminophores, with which the host network is doped, such that a transfer of the excitation energy of this type can take place between several of the luminophores. For example, an excitation energy can be transferred partially from a first to a second luminophore and partly from the second luminophore to a third luminophore, as well as, if necessary, to other luminophores. By correspondingly incomplete transfer of the excitation energy it can be achieved that all these luminophores emit luminescence and the intensities of the luminescence peaks are used to detect the corresponding safety feature.

To manufacture safety features with different codings, for example, to provide different types of documents of value with different codings, various luminescent pigments according to the invention can be used, which have different proportions -P- of the peak intensity of the second luminescence peak in the sum of both peak intensity. While for a first safety feature a luminescent pigment having a first proportion -P- of the second peak intensity is used, other safety features receive the luminescent pigment with a -P- ratio different from the second peak intensity, such that the spectral location of the luminescence peaks is as in the case of the first security feature. Naturally, for the coding of different documents of value security features can also be used which simultaneously contain different or also several of the luminescent pigments according to the invention. For example, the security features may be coded with different types of luminescent pigments according to the invention, whose first and second luminescence peaks are respectively at different wavelengths. Each coding then corresponds to a given combination of proportions -P- of the respective second peak intensities of the different luminescent pigments of the security feature. The security feature according to the invention can have, in addition to one or more luminescent pigments according to the invention, also one or more different luminescent pigments.

The invention also relates to a security element that has a security feature according to the invention. The security element is intended to be applied to a document of value or integrated into a document of value. The security feature is, for example, a security strip, a security thread, a security tape or a transfer element to be applied to a document of value. In addition, the security feature can be mixed with a printing ink provided, for example, for application on a document of value. The printing ink containing the security feature can be printed, for example, in one or several areas determined on the document of value. The security feature can also be integrated into the document of value, for example, by mixing with the substrate material of the document of value, in particular, a substrate of paper or plastic, during its manufacture. The invention also relates to a security paper and a document of value in which the security feature according to the invention is applied or integrated and / or which has a security element provided with the security feature and / or a security ink. printing with the security feature. The safety feature can be mixed with the security paper during the manufacture of security paper. The security feature can be applied in whole or in part, in particular, in the form of symbols or patterns, on a surface of the security document or of the security paper or of the security element. Different sections of the security document or security paper or of the security element can be provided with security features with different coding.

The documents of value to be protected can be, for example, tickets, checks, identity documents, passports, credit cards, bank cards, tickets, bonds, securities, certificates, vouchers, etc.

Next, the invention is explained by way of example on the basis of the following figures. They show:

Figure 1a, a luminescence spectrum of a luminescent pigment with a peak -B- of luminescence of a second luminophore -L2- and a first luminescence, which disappears, of a first luminophore -L1-,

1b, c, respectively the luminescence spectrum of a luminescent pigment according to the invention with a first luminance peak -A- of the first luminophore -L1- and a second luminance peak -B- of a second luminophore -L2-,

Figure 2, the development of the proportion -P- of the peak intensity of the luminescence peak -B- of the second luminophore -L2- in the sum of the peak intensities of the first peak -A- of luminescence and the second peak -B- of luminescence.

The luminescence spectrum of a luminescent pigment which is doped with a first luminophore -L1- and a second luminophore -L2-, between which a complete transfer of its excitation energy takes place, is represented in FIG. The luminescence spectrum is composed of a luminescence peak -B- length -A ^b - wave resulting from the luminescence of the second phosphor L2. The first phosphor acts as activator which transfers its excitation energy to the second luminophore completely and therefore does not show luminescence in length -A ^a - wave, wherein the first luminophore L 1 itself usually emit luminescence. The luminescent pigment of the example of Figure 1a shows the second luminophore -L2- with a molar -y0- molar fraction.

Figures 1b, 1c show respectively the luminescence spectrum of a luminescent pigment according to the invention, which is also doped with the first luminophore -L1- and the second luminophore -L2-. The luminescent pigments of the examples of FIGS. 1b, 1c are characterized, unlike the luminescent pigment of FIG. 1a, by a special fraction-molar, with which the second luminophore -L2- is contained in the luminescent pigment. Both luminescent pigments of FIGS. 1b, 1c differ from each other solely by the molar fraction -and- molar of the second luminophore -L2-, which in the luminescent pigment of FIG. 1b is identified as -y1- and in the example of the Figure 1 c, as -y2-.

Both luminescence spectra of FIGS. 1b, 1c respectively have a luminance peak -A- at a wavelength -A ^to -, resulting from the luminescence of the first luminophore -L1-, and a luminance peak -B- at -A length ^b - wave, resulting from the luminescence of the second phosphor L2. In contrast to the example of figure 1a, in these examples the luminescence of the first luminophore -L1- also appears, although the first luminophore -L1- tends to transfer its excitation energy to the second luminophore -L2-. In the example of figure 1b, the intensity ratio IA / IB of both peaks -A-, -B- of luminescence is approximately 0.43: 1, which corresponds to a ratio P = I ^b / (I ^a + I ^b ) of the intensity -I ^b - of peak in the sum of both intensities (I ^a + I ^b ) of peak of 70%. In the example of figure 1c, the intensity ratio I ^a / I ^b of both peaks -A-, -B- of luminescence is approximately 1.9: 1, which corresponds to a ratio P = I ^b / ( I ^a + I ^b ) of the intensity -I ^b - of peak in the sum of both intensities (I ^a + I ^b ) of peak of 34%.

In FIG. 2, the proportion P = I ^b / (I ^a + I ^b ) of the second peak intensity is represented schematically, as a function of the molar-y-mole fraction of the second luminophore -L2-. In a wide range of the -molar fraction, the proportion -P- of the intensity -I ^b- of the peak is 100%, since for these fractions -and molars of the second luminophore a complete transfer of the excitation energy of the first luminophore -L1- to the second luminophore -L2-. Only for very small fractions and molars, the -P ratio falls below 100% and is continuously reduced to 0% for a molar fraction y = 0, ie when the luminescent pigment only has the first luminophore , but no second luminophore -L2-. For suitable fractions-and-molars of the second luminophore, for example, -y1- and -y2-, then an incomplete transfer of the excitation energy to the second luminophore results. In FIG. 2, with -y0-, -y1- and -y2- the molar fractions -and molars of the second luminophore -L2- are marked in the luminescent pigment for the examples of FIGS. 1a, 1b, 1c.

The luminescent pigment according to the invention shows a qualitative modification of the luminescence spectrum as a function of the molar fraction of the second luminophore, ie, for the luminescent pigment according to the invention, the peak intensities do not scale uniformly, but rather the ratio of the peak intensities of the first and second luminescence peak changes as the molar-y-mole fraction of the second luminophore changes. The special molar fraction -and- of the examples of FIGS. 1b and 1c leads to a considerable change in the ratio of intensities of both peaks -A- and -B- of luminescence. In this example, if the -molar fraction changes, an inverse change of both peak intensities results. While the peak intensity -lA- of the first luminance peak A- for the molar-y1 fraction is smaller than for the molar -y2- fraction, the intensity -lB- of the peak of the second luminance peak -B- for luminescence the -y1- molar fraction is greater than for the -y2- molar fraction. If the -molar fraction of -y1- changes to -y2-, the luminescence peak -A- becomes stronger at the expense of the luminescence peak -B-.

Two specific examples of luminescent pigments according to the invention are indicated below for use in safety features. The luminescent pigments respectively have a host network doped with a first and a second phosphor. As in the example shown above, the first and second luminophore are chosen so that the first luminophore -L1- acts as an activator and tends to transfer its excitation energy to the second luminophore-L2-. However, in the case of fractions-and molars of the second pigment -L2- luminescent according to the invention, an incomplete transfer of the excitation energy results, so that the luminescence spectrum of the respective luminescent pigment has both the peak -B - of luminescence of the second luminophore -L2- as well as the peak -A- of luminescence of the first luminophore -L1-.

Example 1: Y2.68-yHoyYb0.32A¡5O12

A yttrium-aluminum garnet, doped with rare earths, is used as the luminescent pigment. This is doped with the first luminophore -Yb- and the second luminophore -Ho-. In case of excitation with light of a wavelength of 941 nm, the luminescence spectrum of the luminescent pigment has a first peak -A- of luminescence at a wavelength A ^a = 1027 nm, which results from the luminescence of the first luminophore -Yb-, and a second peak -B- of luminescence at a wavelength A ^b = 2086 nm, which results from the luminescence of the second luminophore -Ho-. The dependency that shows the proportion -P- of the intensity -I ^b - of peak -Ho- in the sum I ^a + I ^b of both intensities -I ^a -, -I ^b - of peak with respect to the fraction -and- molar of the second luminophore corresponds qualitatively with the development shown in Figure 2.

Example 1a: Y2.64Hoo, o4Ybo, 32AlsOi2

For the manufacture of this yttrium-aluminum garnet doped with rare earths, 2,308g of Y (NÜ3) 3'6H2Ü, 4,282g of Al (NÜ3) 3-9H2Ü, 0.288g of Yb (NÜ3) 3-5H2Ü, 0.040 are dissolved. g of Ho (NÜ3) 3-5H2Ü and 2,742 g of urea at 60 ° C in 15 g of water and then evaporated at 675 ° C. The parameter -Yb- of concentration of 0.32 and the parameter -Ho- of concentration of 0.04 correspond, due to the empirical formula of 20 atoms, with a fraction -Yb- molar of x = 0.016 and a fraction - Homolar of y = 0.002. For these fractions -x-, -and- molars of the first and the second luminophore, a P = I ^b / (I ^a + I ^b ) ratio of the second peak intensity -I ^b- of about 46% results. That is, in this case, the intensity -I ^to - peak luminescence peak -A- and intensity -I ^b - peak-peak luminescence -B are comparable.

Counterexample not according to the invention of example 1: Y2,58Hoo, iYbo, 32AlsOi2

For the manufacture of this yttrium-aluminum garnet doped with rare earths, 2,256 g of Y (NÜ3) 3'6H2Ü, 4,282g of Al (NÜ3) 3-9H2Ü, 0.288g of Yb (NÜ3) 3-5H2Ü, 0.101 are dissolved. g of Ho (NÜ3) 3-5H2Ü and 2,742 g of urea at 60 ° C in 15 g of water and then evaporated at 675 ° C. The parameter -Yb- of concentration of 0.32 and the parameter -Ho- of concentration of 0.1 correspond, due to the empirical formula of 20 atoms, with a fraction -Yb- molar of x = 0.016 and a fraction - Homolar of y = 0.005. For this luminescent pigment, a P = I ^b / (I ^a + I ^b ) ratio of the second peak intensity -I ^b- of approximately 86% results, which corresponds to a ratio of peak peak intensity -Ho - of luminescence with respect to peak intensity of peak Yb- of luminescence of 6: 1.

Example 2: Na0.9875Er0.0025Ho0.01Ti0.025Nb0.975O3

As a luminescent pigment, a sodium doped niobate with rare earths is used, which is doped with the first luminophore -Er- and the second luminophore -Ho-. For the production of this rare earth doped niobate, 2.808 g of Na2CO3, 6.956 g of Nb2O5, 0.107 g of TiO2, 0.0257 g of Er2Ü3 and 0.101 g of Ho2Ü3 are homogenized in an agate mortar and annealed for 8 hours in a corundum crucible at 1150 ° C. In case of excitation with light of a wavelength of 650 nm, the luminescence spectrum of the luminescent pigment has a first peak -A- of luminescence at a wavelength A ^a = 982 nm resulting from the luminescence of the first luminophore -Er-, and a second peak -B- of luminescence at a wavelength A ^b = 1200 nm, which results from the luminescence of the second luminophore -Ho-. The parameter -Er- of concentration of 0.0025 and the parameter -Ho- of concentration of 0.01 correspond, due to the empirical formula of 5 atoms, with an -Ermolar fraction of x = 0.0005 and a -Ho- molar fraction of y = 0.002. For these fractions -x- molars of the first luminophore -Er- and of the second luminophore -Ho- a proportion P = I ^b / (I ^a + I ^b ) of the second peak intensity -IB- of about 40% results, which corresponds to a ratio of the peak intensity of the peak-Ho- of luminescence to the peak intensity of the peak -Er- of luminescence of 2: 3.

Counterexample not according to the invention of example 2: Na0i8975Er0i0025Ho0i1Ti0i205Nbo, 795O3

As in Example 2, as a luminescent pigment it is considered a sodium niobate doped with rare earths. For the production of this rare earth doped niobate, 2.517 g of Na2CO3, 5.591 g of Nb2O3, 0.866 g of TiO2, 0.0252 g of Er2Ü3 and 0.999 g of Ho2Ü3 are homogenized in an agate mortar and annealed for 8 hours in a corundum crucible at 1150 ° C. The parameter -Er- of concentration of 0.0025 and the parameter -Ho- of concentration of 0.1 correspond, due to the empirical formula of 5 atoms, with an -Ermolar fraction of x = 0.0005 and a -Ho- molar fraction of y = 0.02. For this luminescent pigment, a P = I ^b / (I ^a + I ^b ) ratio of the second peak intensity -I ^b- of approximately 93% results, which corresponds to a ratio of peak peak intensity -Ho - of luminescence with respect to the peak intensity of the luminiscence peak -Er- of 13.3: 1.

Claims (13)

1. Security feature to protect valuable documents, which includes: a luminescent pigment having a host network doped with a first luminophore (L1) and a second luminophore (L2) and which can be excited optically for the emission of luminescent light, such that the mole fraction of the second luminophore in the luminescent pigment is between 50 ppm and 2000 ppm, such that an emission band of the first luminophore overlaps with an absorption band of the second luminophore and the luminescent pigment is realized in such a way that an excitation energy of the first luminophore (L1), generated by optical excitation of the luminescent pigment can be transferred to the second luminophore (L2) by an interaction between the first luminophore (L1) and the second luminophore (L2), characterized - in that the molar fraction (x) of the first luminophore (L1) in the luminescent pigment and the molar fraction (y) of the second luminophore (L2) in the luminescent pigment are chosen so as to achieve an incomplete transfer of the energy of the luminescent pigment. excitation of the first to the second luminophore, such that the luminescent light of the luminescent pigment has a luminescence spectrum with a first peak (A) of luminescence emitted by the first luminophore (L1) and a second peak (B) of luminescence emitted by the second luminance. luminophore (L2), such that the ratio (P) of the peak intensity (IB) of the second peak (B) of luminescence in the sum of the intensities (IA, I ^b ) of the peak of the first (A) and the second peak (B) of luminescence is at least 20% and at most 80%, preferably at least 30% and at most 70%, particularly preferably at least 40% and at most 60% , Y - because the peak wavelengths of the first and the second luminescence peak are shifted to longer wavelengths compared to the optical excitation of the luminescent pigment, and - because the first (L1) and the second luminophore (L2) are chosen from the rare earth ions erbium, holmium, neodymium, thulium, ytterbium, and - because the host network is made as an inorganic host network and because the host network is a host network with a garnet structure or with a perovskite structure or is an oxide or a tungstate or phosphate or niobate or tantalate or silicate or aluminate . Security feature, according to claim 1, characterized in that the peak intensity of the first luminescence peak (A) and the peak intensity of the second luminescence peak (B) have a ratio of intensities to each other that is intrinsically determined by the composition of the luminescent pigment. 3. Security feature according to any of the preceding claims, characterized in that the first and second luminophore (L1, L2) are respectively substantially homogeneously distributed in an area of the volume of the host network, which is doped with both the first (L1), as also with the second luminophore (L2). Security feature, according to any of the preceding claims, characterized in that the luminescent pigment components, in particular, the host network and the first (L1) and the second luminophore (L2), are chosen so that these, in the case of modified molar fractions (and) of the second luminophore, tend to a complete transfer of the excitation energy from the first (L1) to the second luminophore (L2). 5. Security feature, according to any of the preceding claims, characterized in that through a change of the molar fraction (y) of the second luminophore (L2) the intensities can be modified inversely (I ^a , I ^b ) peak of the first and second luminescence peak (A, B). Security feature, according to any of the preceding claims, characterized in that the wavelengths (ÁA, ÁB) of the first and second peak (A, B) of luminescence are spectrally separated from each other at least 20 nm, preferably at least 30 nm. 7. Security feature according to any of the preceding claims, characterized in that the interaction through which the excitation energy of the first luminophore (L1) can be transferred to the second luminophore (L2) takes place in an area of the volume of the host network that is doped with both the first and the second luminophore. 8. Security feature according to any of the preceding claims, characterized in that the excitation energy of the first luminophore (L1) can be transferred through an interaction without radiation of the first luminophore (L1) to the second luminophore (L2). Security feature, according to any of the previous claims, characterized in that the wavelengths (A ^a , A ^b ) of the first wave and the second peak (A, B) of luminescence are in the near infrared spectral range , in particular, in the spectral range between 750 nm and 2900 nm, preferably between 800 nm and 2200 nm. Security element or printing ink having one or more security features according to any of claims 1 to 9. 11. A security document or security document having one or more security features according to any one of claims 1 to 9, and / or a security element according to claim 10, and / or a printing ink, according to the invention. claim 10 12. Method for detecting a security feature, according to any of claims 1-9, wherein - the security feature is irradiated with light of a spectral range in which it absorbs the luminescent pigment of the security feature, to optically excite the luminescent pigment to emit luminescent light, and - intensities of the first and second peak (A, B) of luminescence contained in the luminescence spectrum of luminescent light and - the detected intensities of the first and second peak (A, B) of luminescence are evaluated to detect the safety characteristic. The method according to claim 12, wherein, by irradiating the safety feature, an excitation energy of the first luminophore (L1) is generated, which is partially transferred from the first luminophore (L1) to the second luminophore (L2), such that the excitation energy of the first luminophore (L1) - is generated directly by selective optical excitation of the first luminophore (L1), and / or - is generated by optical excitation of the host network and subsequent transfer of the excitation energy of the host network to the first luminophore (L1).